I really need advice-

My husband is of a different religion- which is a disappointment to me. I am trying to accept this and live our lives around our differences as best we can.
What I can't accept, however, is his insistance on practing his religion with a group of men and women(many single). This is his "group", and they meet for weekend encounters, and he went with some of them on a 5 day session on the other side of the country! He says that he has to "practice" his religion this way to advance in his studies. He has been gone for quite a few weekends in the past 2 years. There are no places in our town for him to practice.

Is it reasonable for me to ask him not to travel with these people? I don't think it's good for our marriage, and it makes me uncomfortable.\

He says I should be happy for him, that when you love someone you are interested in what they are doing. He really resents me for not being more "open"

It has put a major wedge in our marriage.

Any advice?
Thanks for reading.